1. Field of the Invention
This invention relates generally to a cryogenic liquid storage-gas supply system, as for example a portable oxygen system used by individuals for personal breathing.
2. Description of the Prior Art
The transfer of cryogenic liquid from a supply container into a smaller storage-dispensing container such as a portable breathing apparatus must meet the following requirements:
(a) The transfer should be made rapidly. PA1 (b) Losses of cryogenic fluid by vapor generation, should be minimized. PA1 (c) Adequate equilibrium pressure of liquid transferred into the storage-dispensing container should be preserved to facilitate complete withdrawal. PA1 (d) The transfer should be terminated with enough precision to avoid significant underfill or overfill of the storage-dispensing container.
The above requirements must be met with due consideration of factors such as low bulk, weight and cost of component parts in the portable system, and compatibility of such components with cryogenic and oxygen gas service.
Rapid transfer is important for two reasons. First, rapid transfer is desirable in deference to the users' convenience. A practical portable oxygen gas breathing system must be capable of being filled and made ready for use in as short a span of time as possible.
Second, the transfer must be made rapidly in order to minimize loss of cryogenic fluid. Transfer loss is an acute problem with a small portable oxygen gas breathing system because the heat gain per unit volume of liquid transferred tends to increase as the size of the system decreases. Rapid transfer reduces losses because it reduces exposure time of the cold fluid to surfaces in contact with ambient temperature during the transfer operation. Inefficient transfers can often result in more fluid being lost by vaporization than retained in the receiving container for useful employment.
The rate of cryogenic liquid transfer is dependent upon the pressure level of the supply container. A higher pressure affords greater driving force for transport of liquid between containers and for expulsion of vapor from the storage-dispensing container as fill progresses.
The rate of cryogenic liquid transfer is also dependent upon the rate at which vapor is generated during the transfer and upon the rate at which this vapor can be vented from the storage-dispensing container. Vapor generation is caused by the abovementioned heat gain of cryogenic liquid-in-transit, by heat gain of liquid contacting the warmer walls of the storage-dispensing container, and by flash-vaporization of cryogenic liquid passing from a higher pressure to a lower pressure container. The rate at which vapor is vented is limited by the flow resistances in the venting system. Because of such vent-flow resistances, a higher rate of vapor generation will result in a higher pressure in the storage-dispensing container and a consequent reduction of the cryogenic liquid transfer rate into this container. The slower transfer rate in turn results in still greater heat leak into the cryogenic liquid passing through the external conduit system, thereby further aggravating the vapor-generation problem attendant a container filling operation.
The foregoing problem cannot be solved merely by arbitrarily increasing the size of the various components in the venting system in order to reduce their flow resistance. Undue elimination of flow resistance in the venting circuit will shift a substantial part of the overall pressure difference (supply container-to-atmosphere) to the liquid transfer conduit, and will result in excessive loss of pressure in the storage-dispensing container. Whereas a low container pressure will accelerate the cryogenic liquid transfer rate, it will also greatly increase the flash-vaporization of the cryogenic liquid which must now reject sufficient internal heat by vaporization to reach equilibrium at the lower pressure. Thus, the cryogenic fluid transfer losses can become excessive, even with high transfer rate. Moreover, the lower equilibrium pressure of the cryogenic liquid may be inadequate for subsequent dispensing of the gas through a breathing system or other consuming apparatus.
U.S. Pat. Nos. 3,797,262 and 3,864,928 to Eigenbrod disclose a liquid oxygen breathing apparatus which satisfies the basic requirements for such a system. The portable storage-dispensing container is closecoupled to the reservoir or supply container, which minimizes the mass and surface area of warm material contacted by the cryogenic liquid in the conduit system, thereby reducing warm-up of the cryogenic liquid conducted through the conduit system during transfer. A vapor venting system is used with the storage-dispensing container which contains minimum flow restriction, consistent with pressure-retention requirements for the cryogenic liquid stored in the portable container. These features afford rapid transfer of cryogenic liquid between the supply and storage-dispensing containers. Moreover, the proper flow restriction can be provided in the vapor venting system so as to retain adequate pressure in the storage--dispensing container yet permit rapid expulsion of vapor from this container, such pressure difference representing a desirable combination of rapid cryogenic liquid transfer and reduced flash-vaporization of liquid.
Eigenbrod U.S. Pat. Nos. 3,797,262 and 3,864,928 disclose an automatic cryogenic liquid fill termination valve which operates in response to the appearance of liquid in the vent fluid. The presence of cryogenic liquid is sensed by temperature or pressure change in the venting system upstream the vent valve, and a signal is generated and transmitted through an external circuit (electrical or pneumatic) to the valve.
Although the portable oxygen storage-dispenser container of Eigenbrod U.S. Pat. Nos. 3,797,262 and 3,864,928 can be filled reliably, repeatedly and dependably without overfilling or discharging cryogenic liquid, the fill-termination controls account for appreciable fractions of the total weight, bulk and cost of the system. The automatic vent valve which closes upon a signal from the sensor is disclosed as either a four-way, pressure-operated valve or an electric solenoid valve. The former valve is a rather complex, expensive device requiring rigid specifications and meticulous quality control to insure satisfactory performance. The latter valve additionally requires a thermistor or similar sensor, a relay controller and a source of electric power.
An object of this invention is to provide an improved apparatus for terminating the flow of cryogenic liquid from a supply container into storage-dispensing container.
Another object is to provide such improved apparatus which is reliable, very compact, lightweight and low cost.
Other objects and advantages will be apparent from the ensuing disclosure and appended claims.